"It's wrong to say it is a settled matter because it's absolutely not, it's not a settled matter. That means, you know a lot of people think you have to be born here," Trump said at rally in New Hampshire before a lethargic and relatively small midday crowd.

Trump continued planting doubts about Ted Cruz's citizenship, saying GOP voters will render the final verdict.

"I mean it's one of those little decisions. I'm sure Ted is thrilled that I'm helping him out but I am. I mean I am. I mean he's gotta go and he's got to fix it."

Cruz has dismissed Trump's suggestions, saying he's "never breathed a breath of air on this planet when I wasn't an American citizen by virtue of being born."

A new poll shows Trump with a commanding lead in New Hampshire, with John Kasich, Marco Rubio and Cruz in a virtual tie for second. But Trump's camp fears a Cruz victory in Iowa would catapult him ahead in New Hampshire.

Gov. Christie: “I never donated to Planned Parenthood”

Rubio is also looking over his shoulder at Chris Christie, who is trailing him by just four points in New Hampshire. On Monday, Rubio attacked Christie on key conservative issues.

"Our next president cannot be someone who supports Common Core, who supports gun control, who's personally contributed to Planned Parenthood. These are things we need to be reversing," said Rubio.